Heretofore, if a constricted portion is produced in the blood vessel, such as artery, a balloon forming portion annexed to near the distal end of a catheter is introduced into the constricted portion in the blood vessel to from a balloon to expand the constricted portion to improve the blood stream, by way of performing a percutaneous blood vessel forming technique (PTA).
Meanwhile, it is known that, if the PTA is applied, the portion which once suffered from constriction tends to undergo re-constriction with a high probability.
For preventing this re-constriction, it is practiced to apply a tubular stent in the portion of the vessel treated with PTA. This stent is buried in an expanded state in a blood vessel 2 as shown in FIG. 11 to support the blood vessel 2 from its inside to prevent occurrence of re-constriction in the blood vessel 2.
In clinical cases in which a stent prepared by weaving a linear material of stainless steel in a mesh is introduced in the portion of the vessel treated with PTA, re-constriction occurred in a probability of approximately 15%.
For prohibiting this re-constriction, there is proposed in Japanese Laying-open publication Hei-5-502179 (WO91/01097) a stent prepared by polymer fibers containing a pharmaceutical capable of preventing the constriction from occurring.
Meanwhile, blood re-constriction after loading the stent occurs in a majority of cases at a stent end or beginning from the stent end.
The stent used in the inserted state in the blood vessel holds the blood vessel in the expanded state, so that it is designed to have tenacity sufficiently higher than that of the blood vessel. For example, the blood vessel has a Young's modulus equal to approximately 3.times.10.sup.7 pascal, whereas the stainless steel as a main material of the stent used for holding the blood vessel in the expanded state is approximately 3.times.10.sup.11 pascal.
At this time, the blood vessel 2 is reduced in diameter at the portions in register with ends 1a, 1b of the stent 1 where the blood vessel 2 ceases to be supported by the stent 1. The portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1 represent stress-concentrated portions.
The blood vessel 2, especially the artery, perpetually performs pulsations to cause the blood to flow. The result is that the load due to the pulsations are repeatedly applied to the stress-concentrated portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1, so that the inner wall of the blood vessel 2 tends to be damaged by the ends 1a, 1b of the stent 1. Since the load is applied to the stress-concentrated portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1, there are produced injuries in the inner film or beginning from the inner film to the outer film of the blood vessel 2. If damaged, the blood vessel 2 recuperates the damaged portion as a reaction proper to a living body. In recuperating the damaged portion, the blood vessel 2 has its inner film multiplied excessively to cause the re-constriction.